Among the various reactor designs, there are gas-cooled piled-ball nuclear reactors whose core can comprise, in a vessel forming side, bottom and top reflectors, a piled mass of nuclear-pile elements in the form of balls. The latter can be composed of graphite and can contain uranium or thorium or oxides or other compounds thereof directly or in the form of ceramic-like particles embedded in the graphite balls.
A conventional cooling system having a gas-cooled reactor of this type has a cooling gas inlet at the top of the reactor, a cooling gas outlet at the bottom of the reactor, and a blower, compressor or other means for displacing the gas which forms the primary coolant because it is forced directly through the mass of balls. The gas passes from the outlet through a main heat exchanger or gas cooler in which the gas can be cooled by indirect heat exchange with a secondary coolant, the latter being cooled by a tertiary coolant or serving directly as a power-transfer agent if, for example, the secondary coolant is water which is converted to steam and used to drive an electricity-generating turbine.
When a reactor of this type is shut down, it is common practice to close down the main cooling system. Nevertheless, residual fission heat must be dissipated for the duration of the shutdown, i.e. frequently for relatively long periods.
It is thus imperative to provide the reactor with means for removing the after heat, i.e. the heat resulting from nuclear decay in a shut-down reactor of the aforedescribed type when the primary coolant system is cut out.
Thus is has already been proposed to provide an after-heat auxiliary cooling gas cycle in parallel to the main cooling gas circulation, referred to hereinafter as the operating loop. The operating loop and the after-heat removal system are coupled together at the cooling gas inlet and the cooling gas outlet of the reactor core.
With such systems, however, it is difficult to reliably throttle the flow of the auxiliary gas stream through the operating loop or to utilize reliably the components of the primary cooling system to dissipate the after heat.
Bypasses and the like have been provided in the operating loop but these have not proved to be fully effective because they were incapable of preventing the entrainment of corrosive media from the operating loop into the reactor core which promoted graphite corrosion.
Mention may also be made of the fact that the operating loop, especially for high-temperature reactors, generally includes gas turbines, ducts of large-flow cross section, adjustable turbine blade arrangements, multiple cooling gas paths and the like which cannot be readily adjusted for after-heat removal and do not lend themselves to operation with the more limited gas flow rates for after-heat removal.
For example, German patent document No. 27 13 463 provides parallel operating loops with partitions separating them in the cooling-gas chambers which have not been utilized effectively for after-gas cooling.
With high-temperature reactors having gas turbines as described in German patent documents No. 21 59 696 and 24 40 140, the after-heat removal can be effected by utilizing operating loops which are not shut down, by opening a bypass in the working gas circulating path or by operating a secondary water emergency cooling gas circulation.
With this arrangement as well the after-heat removal system is coupled with the operating loop and/or emergency cooling system so that flow cross sections must be provided for the cooling gas which are not fully effective for the rapid removal of this after heat.
it is obviously desirable to remove the after heat at as low a temperature as is possible with a minimum gas flow and this cannot be accomplished readily when the operating loop is coupled with the after-cooling system and components of the operating loop, dimensioned for the primary coolant must be utilized for removal of the after heat. Furthermore, when the after cooling is effected utilizing the emergency cooling system, the latter must be provided at increased cost so that it can operate at the high temperatures which may be present in the cooling gas utilized for the removal of the after heat.